This invention relates to integrated circuit fabrication tools and processes and, more particularly, to a method and apparatus for cleaning a pellicled reticle.
Integrated circuits (IC) commonly are fabricated on a semiconductor wafer. The semiconductor wafer typically is subjected to doping, deposition, etching, planarizing and lithographic processes to form semiconductor devices in the wafer. The wafer typically is cut to form multiple semiconductor xe2x80x9cIC chipsxe2x80x9d. Each chip includes many semiconductor devices. Although the label semiconductor is used, the devices are fabricated from various materials, including electrical conductors (e.g., aluminum, tungsten), electrical semiconductors (e.g., silicon) and electrical non-conductors (e.g., silicon dioxide).
A reticle is used in a lithographic process to define a photomask. A lithographic process refers to a process in which a pattern is delineated in a layer of material (e.g., photoresist) sensitive to photons, electrons or ions. The principle is similar to that of a photocamera in which an object is imaged on a photo-sensitive emulsion film. While with a photo-camera the xe2x80x9cfinal productxe2x80x9d is the printed image, the image in the semiconductor process context typically is an intermediate pattern which defines regions where material is deposited or removed. The lithographic process typically involves multiple exposing and developing steps, wherein at a given step the photoresist is exposed to photons, electrons or ions, then developed to remove one of either the exposed or unexposed portions of photoresist. Complex patterns typically require multiple exposure and development steps.
A typical lithographic system includes a light source, optical system and transparent photomask. The light source emits light through the optical system and photomask onto a photoresist layer of a semiconductor wafer. The photomask defines the xe2x80x9cintermediate patternxe2x80x9d used for determining where photoresist is to be removed or left in place. Conventional photomasks are transparent masks. A photomask typically is formed on a glass blank. The mask and blank together are referred to as a reticle. Conventional materials for the blank include soda lime, borosilicate glass or fused silica. The photomask is formed by a film of opaque material. Typically, the film is formed with chrome less than 100 nm thick and covered with an anti-reflective coating such as chrome oxide. The purpose of the anti-reflective coating is to suppress ghost images from the light reflected by the opaque material.
The photomask serves to define geometries for materials deposited or etched on the wafer or materials applied to the wafer. The patterned film on the reticle blank includes mask lines and line spacings of less than 10 microns. Depending on the reduction factor x, line width and line space geometries for a resulting semiconductor device are from less than 10 microns to less than 2 microns. Other mask line spacings and semiconductor line spacings also can be achieved. When working with such small geometries, it is important that the reticle and other components in the fabrication processes be free of foreign particles. A tiny speck of dust alters the desired pattern to be imaged onto the wafer. Conventionally, a thin transparent membrane, referred to as a pellicle membrane, is applied over the photomask portion of the reticle to keep the photomask portion free of foreign particles. The pellicle membrane typically is positioned at a height above the photomask. Such height is greater than the focal length of the light imaged onto the photomask. Thus, small particles on the pellicle membrane will not block light from reaching the photomask.
Another problem caused by foreign particles is bad registration of the reticle. During a lithographic process, the reticle rests on a reticle table. The reticle table typically is part of a stepper device which also includes a light source and a stepper control. The stepper control manages the relative position of the light source and the reticle table. Even the smallest of particles on the edge of the reticle can lift a portion of the reticle off the reticle table. Such offset of the reticle can result in bad registration of the light onto the wafer, which in turn can result in bad overlay from one pattern to another. Because the pellicle membrane typically is very fragile, the pellicle membrane is destroyed during the course of cleaning the reticle. Conventionally, the pellicle membrane is removed, the entire surface of the reticle is cleaned, and then the reticle undergoes requalification. Such a process is very time consuming and costly. Accordingly, there is a need for an alternative method and apparatus for cleaning a reticle.
According to the invention, a reticle having a pellicle is cleaned without removing or damaging the pellicle. The pellicle includes a pellicle membrane and a pellicle frame. A cover encases the pellicle, sealing it from the external environment during the cleaning process.
According to one aspect of the invention, the cover fits around the periphery of the pellicle frame and covers the pellicle membrane. An edge of the cover in contact with the reticle (adjacent the pellicle frame) forms a seal. In a preferred embodiment, the edge includes a groove within which is an O-ring seal.
According to another aspect of the invention, the reticle is fastened to reticle supports on a spin chuck during the cleaning process. An anchor plate presses the cover to the reticle, maintaining the pellicle sealed from the external environment. The anchor plate fastens to the spin chuck. Thus, the cover and reticle are sandwiched together between the anchor plate and spin chuck.
According to another aspect of the invention, a system for cleaning a reticle having a pellicle frame and pellicle membrane is provided. The reticle has a pattern formed on a first surface, the pattern occurring within a first area of the first surface. The pellicle frame is attached to the first surface, defining a border of the pattern. The pellicle membrane is attached to the pellicle frame and elevated above the pattern, the pellicle membrane sealing the first area. The reticle is secured to a support. A lid encases the pellicle frame and pellicle membrane. The lid has a first surface in contact with the first surface of the reticle. A clamp pushes the lid to the reticle.
According to another aspect of the invention, the lid has a first surface in contact with the first surface of the reticle. The lid has a groove formed within the lid""s first surface. An O-ring seal is within the groove. The O-ring seal is pressed into contact with the reticle by the lid under a force of the clamp. The clamp is secured to the support.
According to another aspect of the invention, a fluid under pressure is ejected onto the reticle, wherein the pellicle membrane is shielded from the fluid by the lid. A drive mechanism rotates the support, altering a portion of the reticle receiving the fluid under pressure.
According to another aspect of the invention, the lid and reticle serve as an apparatus for encasing the pellicle. The lid has a recessed area which is bordered peripherally by a first wall. The first wall is adjacent to a first edge. The first edge has a seal extending around a peripheral border of the recessed area. The first wall has a height greater than a height of the pellicle frame. The lid encases the pellicle membrane and pellicle frame within the recessed area with the seal making contact with the reticle on the first surface.
According to another aspect of the invention, the seal is an O-ring seal within a groove along the first edge for sealing the recessed area of the lid and the enclosed pellicle frame and pellicle membrane from an environment of the reticle.
According to another aspect of the invention, a method for cleaning a reticle without damaging or removing a pellicle membrane is performed. At one step, the pellicle is covered with a lid to separate the pellicle from an external environment of an uncovered portion of the reticle. At another step, a force is applied to the lid to seal the pellicle from the external environment. At another step, the reticle is secured to a base. At another step, fluid under pressure is ejected onto the uncovered portion of the reticle to clean the uncovered portion of foreign particles.
An advantage of the invention is that a reticle which does not accurately rest on a stepper table due to foreign particles is cleaned without removing or damaging the pellicle. An effect of this advantage is that the reticle does not need to go through an extensive process of re-applying a pellicle frame and pellicle membrane and requalifying the reticle for use in a lithographic process. These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.